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Blood gas and acid base evaluation. Chapter 20. Step one. Normal Ph 7.35 – 7.45 PCO2 35 – 45 mmHg ( 31 – 39 ) PO2 60 – 100 mmHg BE -2 - +2 HCO3 24 AG 8 – 16 Na , K , Cl Ph = pKa + log X HCO3/PCO2 x Solubility A-a gradient N < 20 mmHg O2 – Hb dissociation scale. Step 1.

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## Blood gas and acid base evaluation

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**Blood gas and acid base evaluation**Chapter 20**Step one**• Normal • Ph 7.35 – 7.45 • PCO2 35 – 45 mmHg ( 31 – 39 ) • PO2 60 – 100 mmHg • BE -2 - +2 • HCO3 24 • AG 8 – 16 • Na , K , Cl • Ph = pKa + log X HCO3/PCO2 x Solubility • A-a gradient N < 20 mmHg • O2 – Hb dissociation scale**Step 1**• Ph = pKa + log X HCO3/PCO2 x Solubility • CO2 + H2O – H2CO3 – H + HCO3 • A-a gradient N < 20 mmHg • O2 – Hb dissociation scale**Step two**• Is there an OXYGENATION problem • PO2 • Saturation • Must always be interpreted with FiO2 • PF Ratio = PO2/FiO2 • Normal > 400 • ALI < 300 • ARDS < 200**Step 2**• Reason for oxygenation problem • Work out A-a gradient • Decide if it is an intrinsic lung problem**Step 3**• Is there a VENTILATION problem • Look at PaCO2 • <35 hyperventilation • >45 hyperventilation • Remember that ventilation always interpreted with pH in mind**Step 4**• What is the pH • Normal • Normal • Compensated • Mixed • Alkalosis • Acidosis**Step 5**• Does the PaCO2 FIT IN with the pH • Alkalosis + PaCO2 < 40 mmHg = Respiratory Alkalosis • Acidosis + PaCO2 > 40 mmHg = Respiratory acidosis • If the PaCO2 does not fit in with the ph , move to next step • If the PaCO2 fit in with Ph = PRIMARY ABNORMALITY**Step 6**• Look at the HCO3 • Does the SBC fit in with the pH • Alkalosis + SBC > 24 = Metabolic alkalosis • Acidosis + SBC < 24 = Metabolic acidosis • This will be the primary abnormality if the PaCO2 did not fit in with pH**But wait , there is more**• Remember that basic 4 abnormalities are • Resp acidosis • Metab acidosis • Resp alkalosis • Metab alkalosis • Uncompensated disorders • Compensated disorders • Mixed disorders • Chronic versus acute**Step 7**• Determine the degree of COMPENSATION • Rules apply ( tables ) • Thus the body always tries to compensate for the primary abnormality • If compensation as expected = normal compensation • If compensation not as expected = mixed disorder**Step 8**• Calculate the ANION GAP if metabolic acidosis • AG = Cations – Anions = 8-16 mEq/L = unmeasured anions • AG = (Sodium + Potasium) – (Choride + Bicarbonate) • Acidosis + AG > 16 Mm = High anion gap metabolic acidosis • Acidosis + AG normal = Low anion gap metabolic acidosis**Step 9**• Look at the Haemotological variables • Haematocrit • Haemoglobin • Abnormal haemoglobins**Step 10**• Look at the metabolites and electrolytes • Lactate • Glucose • Calcium • Sodium • Potassium • Magnesium**Intracellular pH**• 6.8 • Neutral at body temp of 37 degrees celcius • Hydrogen = Hydroxyl • Dissociation of water and other electrlytes**Administration of 8.4% NaHCO3**• Rather treat the cause • Ph < 7.1 metabolic acidosis • Acidific toxins etc to aid renal excretion • Hypernatraemia , hyperosmolality , fluid overload**Chloride/Sodium Ratio**• 0.74-0.78 • > in hyperchloraemic metabolic acidosis • 0.9% Saline , Voluven , Venofundin**Sodium**• Hyponatraemia • Hypervolaemichyponatraemia • Normovolaemichyponatraemia • Hypovolaemichyponatraemia • < 130 Mm – No elective surgery • < 120 Mm – Neurologic derangements • Central pontinedemyelinization if corrected too fast**Sodium**• Hypernatraemia • Hypervolaemic • Normovolaemic • Hypovolaemic • > 150 no elective surgery • Brain oedema if corrected too fast**Potassium**• Hypokalaemia • < 3,5 • Causes • Redistribution, Decreased intake, Increased losses • Symptoms and signs • Cardiovascular , musculoskeletal

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